Crosslinking systems for effecting cure of emulsion polymers are used to provide nonwoven articles, particularly cellulosic webs such as paper towels, with some desired property such as water or solvent resistance. Most crosslinking systems for emulsion polymers which are employed today require temperatures in excess of 100.degree. C. to ensure the development of a decently cured system. While high temperature cures may be acceptable for many applications, such temperatures may be unacceptable in other applications because of an unsuitability of certain types of substrates, operational difficulties, and lastly, they may represent economic hardship due to the high cost of energy.
In the manufacture of paper towels by the double recreping process (DRC process) that deficiency is even more profound. In the DRC process, a basestock of paper is printed on one side with a polymeric binder, flash dried, creped, and printed on the second side, flash dried, and recreped and collected on a roller into a ream of paper. These line rolls run at over 1500 ft/minute. The current process requires a bank of dryers before collecting to cure the binder and prevent blocking, i.e., the tendency of one sheet to stick to an upper or lower layer. The industry wishes to move away from the use of a cure oven and its inherent cost of capital and energy. To make this practical, the binder must cure at ambient condition, i.e., it must cure in an extremely short time, e.g., within a second to 2 minutes, rather than the weeks required for curing vinyl trisisopropoxy silane (VTIPS).
One type of crosslinking system employed for polymeric binders includes a crosslinking mechanism based upon the use of pendent acetoacetate functionality such as that derived by the polymerization of acetoacetoxyethyl methacrylate (AAEM) into the polymer and a polyfunctional reactant therewith. The acetoacetate containing polymer then can be reacted with a multi-primary amine functional moiety, for example, to effect crosslinking. This combination has a very short pot-life and often requires the addition of a blocking agent which tend to severely retard cure.
Another type of crosslinking functionality for polymeric binders is based upon the reaction of carboxyl functionality and a polyaziridine.
The following patents are representative of acetoacetate chemistry in the crosslinking of polymeric emulsions.
U.S. Pat. No. 5,534,310 discloses a method for improving adhesive durable coatings on weathered substrates. The durable coatings are based upon latex binders formed by the polymerization of acrylic and methacrylic esters, such as methyl methacrylate, ethyl acrylate, butyl acrylate, 2-ethylhexyl acrylate, etc., along with vinyl monomers and the like. Durability is enhanced by incorporating acetoacetate functionality into the polymer, typically by polymerization of monomers such as acetoacetoxyethyl methacrylate, acetoacetoxyethyl acrylate (AAEA), allyl acetoacetate, and vinyl acetoacetate. Enamine functionality is incorporated into the polymer for improving adhesion by reaction of the latex containing the acetoacetate functionality with ammonia or an amine.
U.S. Pat. No. 5,426,129 discloses a coating or impregnating composition based on a vinyl addition polymer containing acetoacetate groupings or an enamine. The vinyl addition polymers are based upon the polymerization of a variety of monomers including acrylic and methacrylic acid esters and ethylenically unsaturated monomers such as vinyl acetate, vinyl chloride, etc. A reactive-coalescent is incorporated into the polymer, and these coalescents include monomers such as acetoacetoxyethyl methacrylate and the corresponding enamines which are obtained by reaction with ammonia or ethanolamine.
U.S. Pat. No. 5,451,653 discloses a curable crosslinking system based upon an aldimine/acetoacetate crosslinker. The polymer is a water-based, crosslinkable polymer having utility in industry as a coating or adhesive and is based on the polymerization of a variety of monomers including acrylic and methacrylic acid esters as well as vinyl acetate and other ethylenically unsaturated monomers. Acetoacetate functionality is incorporated into the water-based, crosslinkable polymer by one of two techniques, the preferred being the incorporation via polymerization of acetoacetoxyethyl methacrylate. The acetoacetate functionality is crosslinked by reaction with an aldimine formed by the reaction of an aldehyde and an amine.
A publication by Kodak regarding acetoacetoxyethyl methacrylate and acetoacetyl chemistry discloses the synthesis of polymer systems incorporating acetoacetoxyethyl methacrylate for decreasing solution viscosity and lowering glass transition temperature as well as providing a mechanism for crosslinking the polymer systems. A variety of reactions of acetoacetylated containing polymers is shown as, for example, reaction of a polymer having pendent acetoacetate functionality with melamine, an isocyanate, an aldehyde, or an electron-deficient olefin through a Michael reaction.
U.S. Pat. No. 5,605,953 discloses polymeric systems incorporating both acetoacetoxy functional and amine functional moieties as well as acetoacetoxy and acid functional moieties for providing crosslinked coatings and films. Crosslinking is effected through the use of amines.
The following patents describe crosslinking systems based upon polyfunctional aziridines.
U.S. Pat. No. 4,645,789 discloses the use of highly crosslinked polyelectrolytes for use in diapers and dressings which are based upon acrylic acid-acrylate copolymers, acrylic acid-acrylamide copolymers, acrylic acid and vinyl acetate copolymers, and so forth. Preferred aziridines include the triaziridines based upon trimethylolpropane tripropionates, tris(1-aziridinyl)phosphine oxide, and tris(1-aziridinyl)-phosphine sulfide.
U.S. Pat. No. 4,605,698 discloses the use of polyfunctional aziridines in crosslinking applications. One type of polyaziridine is based upon the reaction of ethylene imine with acrylates of an alkoxylated trimethylolpropane or other polyol. Vinyl acetate/carboxylated urethanes and styrene/acrylics are shown as being crosslinked with polyfunctional aziridines to produce coatings having a low temperature crosslinking functionality.
U.S. Pat. No. 4,278,578 discloses coating compositions for plastic substrates based upon carboxy functional acrylic copolymers which are crosslinked with from about 0.2 to 3% of a polyfunctional aziridine. Carboxy functional acrylic and methacrylic copolymers are for use in maintaining the appearance of wooden floors and the durability of vinyl and other resilient floor coverings. The crosslinking agents are used for effecting crosslinking of the acrylic and carboxyl functional copolymers. Examples include N-aminoethyl-N-aziridylethylamine with a most preferred aziridine being a trifunctional aziridine having equivalent weight of 156 atomic mass units sold under the trademark designation Neocryl CX100 by Polyvinyl Chemical Industries (now by Zeneca Corporation).
U.S. Pat. No. 3,806,498 discloses the use of (1-aziridinyl)alkyl curing agents for acid-terminated polymers. A wide variety of polymers having terminal-free acid groups are described as being crosslinkable through the use of the (1-aziridinyl)alkyl curing agents, and these include those formed by the reaction of esters of carboxylic saturated and unsaturated acids with aziridinyl alcohols.